1. Field of the Invention
The present invention relates to integrating III-V semiconductor devices upon silicon substrates. More particularly this invention relates to the buffer layer between a III-V semiconductor device and a silicon substrate.
2. Discussion of Related Art
A variety of electronic and optoelectronic devices can be enabled by developing thin film relaxed lattice constant III-V semiconductors on elemental silicon (Si) substrates. Surface layers capable of achieving the performance advantages of III-V materials may host a variety of high performance electronic devices such as CMOS and quantum well (QW) transistors fabricated from extreme high mobility materials such as, but not limited to, indium antimonide (InSb), indium gallium arsenide (InxGa1-xAs) (x >0.53) and indium arsenide (InAs). Optical devices such as lasers, detectors and photovoltaics may also be fabricated from various other direct band gap materials, such as, but not limited to, gallium arsenide (GaAs) and indium gallium arsenide (InGaAs). These devices can be further enhanced by monolithically integrating them with conventional devices of silicon and use of a silicon substrate has the additional advantage of cost reduction.
Despite all these advantages, the growth of III-V materials upon silicon substrates presents many challenges. Crystal defects are generated by lattice mismatch, polar-on-nonpolar mismatch and thermal mismatch between the III-V semiconductor epitaxial layer and the silicon semiconductor substrate. When the lattice mismatch between the epitaxial layer and substrate exceeds a few percent, the strain induced by the mismatch becomes too great and defects are generated in the epitaxial layer when the epitaxial film relaxes. Once the film thickness is greater than the critical thickness (film is strained below this thickness and relaxed above this thickness), the strain is relaxed by creating misfit dislocations at the film and substrate interface as well as in the epitaxial film. The epitaxial crystal defects are typically in the form of threading dislocations, stacking faults and twins (periodicity breaks where one portion of the lattice is a mirror image of another). Many defects, particularly threading dislocations, tend to propagate into the “device layer” where the semiconductor device is fabricated. Generally, the severity of defect generation correlates to the amount of lattice mismatch between the III-V semiconductor and the silicon substrate. For these reasons, the large lattice mismatch (approximately 19.2% between the exemplary indium antimonide (InSb) and silicon (Si) combination) typically results in an epitaxial device layer having a high defect density, on the order of 1×109 cm−2 to 1×10 cm−2. The high defect density reduces the carrier mobility theoretically possible in bulk InSb, eliminating many of the technical advantages of “InSb-on-silicon” integration. For example, the electron mobility in bulk InSb films is estimated to be approximately 76,000 cm2/Vs. However, to date, the best reported electron mobility of an InSb film formed over a silicon substrate is significantly lower, approximately 40,000-50,000 cm2/Vs.
Similarly, a high defect density is also detrimental to photonic devices formed in or upon III-V semiconductor device layers on silicon substrates. The recombination-generation (R-G) energies of crystal defects are typically mid-gap, detracting from the performance of a semiconductor device layer that has been band gap engineered for a particular optical wavelength.
Various buffer layers have been used in attempts to relieve the strain induced by the lattice mismatch between the silicon substrate and the III-V device layer and thereby reduce the detrimental defect density of the device layer. For example, as shown in apparatus 100 of FIG. 1A, a material forms a buffer layer 170 between a silicon substrate 110 and a III-V device layer 180. A semiconductor device 190 is then fabricated in or upon device layer 180. Various materials have been utilized as the buffer layer 170. For example, an aluminum antimonide (AlSb) buffer layer 170 has been attempted as has a strontium titanate (SrTiO3) buffer layer 170 between a silicon substrate 110 and a III-V device layer 180. In practice however, as depicted in FIG. 1B, these buffer layers are unable to prevent twins 171, threading dislocations 173 and stacking faults 175 from propagating into the III-V device layer 180 as twins 181, threading dislocations 183, and stacking faults 185. Thus, there remains a need for a buffer layer architecture that enables lower defect density III-V semiconductor device layers formed upon silicon substrates.